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<div class="title">Reading and Writing Attributes </div>  </div>
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<div class="textblock"><h2>Goal </h2>
<p>This tutorial shows you:</p><ul>
<li>How to write attributes?</li>
<li>How to read attributes?</li>
</ul>
<dl class="section note"><dt>Note</dt><dd>Although attributes can be associated with groups and datasets, only attributes with the root group are implemented in OpenCV. Supported attribute types are <code>int</code>, <code>double</code>, <code><a class="el" href="../../dc/d84/group__core__basic.html#ga1f6634802eeadfd7245bc75cf3e216c2">cv::String</a></code> and <code><a class="el" href="../../dc/d84/group__core__basic.html#ga353a9de602fe76c709e12074a6f362ba">cv::InputArray</a></code> (only for continuous arrays).</dd></dl>
<h2>Source Code </h2>
<p>The following code demonstrates reading and writing attributes inside the root group with data types <code><a class="el" href="../../d3/d63/classcv_1_1Mat.html" title="n-dimensional dense array class ">cv::Mat</a></code>, <code><a class="el" href="../../dc/d84/group__core__basic.html#ga1f6634802eeadfd7245bc75cf3e216c2">cv::String</a></code>, <code>int</code> and <code>double</code>.</p>
<p>You can download the code from <a href="https://github.com/opencv/opencv_contrib/tree/master/modules/hdf/samples/read_write_attributes.cpp">here</a> or find it in the file <code>modules/hdf/samples/read_write_attributes.cpp</code> of the opencv_contrib source code library.</p>
<div class="fragment"><div class="line"><span class="preprocessor">#include &lt;iostream&gt;</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="../../d0/d9c/core_2include_2opencv2_2core_8hpp.html">opencv2/core.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="../../d7/d55/hdf_8hpp.html">opencv2/hdf.hpp</a>&gt;</span></div><div class="line"></div><div class="line"><span class="keyword">using namespace </span><a class="code" href="../../d2/d75/namespacecv.html">cv</a>;</div><div class="line"></div><div class="line"><span class="keyword">static</span> <span class="keywordtype">void</span> read_write_attributes()</div><div class="line">{</div><div class="line">    <a class="code" href="../../dc/d84/group__core__basic.html#ga1f6634802eeadfd7245bc75cf3e216c2">String</a> filename = <span class="stringliteral">&quot;attributes.h5&quot;</span>;</div><div class="line"></div><div class="line">    <a class="code" href="../../dc/d84/group__core__basic.html#ga6395ca871a678020c4a31fadf7e8cc63">Ptr&lt;hdf::HDF5&gt;</a> h5io = <a class="code" href="../../d6/dee/group__hdf5.html#ga243d7e303690af3c5c3686ca5785205e">hdf::open</a>(filename);</div><div class="line"></div><div class="line">    <a class="code" href="../../dc/d84/group__core__basic.html#ga1f6634802eeadfd7245bc75cf3e216c2">String</a> attr_mat_name = <span class="stringliteral">&quot;array attribute&quot;</span>;</div><div class="line">    <a class="code" href="../../d3/d63/classcv_1_1Mat.html">Mat</a> attr_mat;</div><div class="line">    attr_mat = (<a class="code" href="../../df/dfc/classcv_1_1Mat__.html">cv::Mat_&lt;float&gt;</a>(2, 3) &lt;&lt; 0, 1, 2, 3, 4, 5, 6);</div><div class="line">    <span class="keywordflow">if</span> (!h5io-&gt;atexists(attr_mat_name))</div><div class="line">        h5io-&gt;atwrite(attr_mat, attr_mat_name);</div><div class="line"></div><div class="line">    <a class="code" href="../../dc/d84/group__core__basic.html#ga1f6634802eeadfd7245bc75cf3e216c2">String</a> attr_str_name = <span class="stringliteral">&quot;string attribute&quot;</span>;</div><div class="line">    <a class="code" href="../../dc/d84/group__core__basic.html#ga1f6634802eeadfd7245bc75cf3e216c2">String</a> attr_str = <span class="stringliteral">&quot;Hello HDF5 from OpenCV!&quot;</span>;</div><div class="line">    <span class="keywordflow">if</span> (!h5io-&gt;atexists(attr_str_name))</div><div class="line">        h5io-&gt;atwrite(attr_str, attr_str_name);</div><div class="line"></div><div class="line">    <a class="code" href="../../dc/d84/group__core__basic.html#ga1f6634802eeadfd7245bc75cf3e216c2">String</a> attr_int_name = <span class="stringliteral">&quot;int attribute&quot;</span>;</div><div class="line">    <span class="keywordtype">int</span> attr_int = 123456;</div><div class="line">    <span class="keywordflow">if</span> (!h5io-&gt;atexists(attr_int_name))</div><div class="line">        h5io-&gt;atwrite(attr_int, attr_int_name);</div><div class="line"></div><div class="line">    <a class="code" href="../../dc/d84/group__core__basic.html#ga1f6634802eeadfd7245bc75cf3e216c2">String</a> attr_double_name = <span class="stringliteral">&quot;double attribute&quot;</span>;</div><div class="line">    <span class="keywordtype">double</span> attr_double = 45678.123;</div><div class="line">    <span class="keywordflow">if</span> (!h5io-&gt;atexists(attr_double_name))</div><div class="line">        h5io-&gt;atwrite(attr_double, attr_double_name);</div><div class="line"></div><div class="line">    <span class="comment">// read attributes</span></div><div class="line">    <a class="code" href="../../d3/d63/classcv_1_1Mat.html">Mat</a> expected_attr_mat;</div><div class="line">    <span class="keywordtype">int</span> expected_attr_int;</div><div class="line">    <span class="keywordtype">double</span> expected_attr_double;</div><div class="line"></div><div class="line">    <a class="code" href="../../dc/d84/group__core__basic.html#ga1f6634802eeadfd7245bc75cf3e216c2">String</a> expected_attr_str;</div><div class="line">    h5io-&gt;atread(&amp;expected_attr_str, attr_str_name);</div><div class="line"></div><div class="line">    h5io-&gt;atread(expected_attr_mat, attr_mat_name);</div><div class="line">    h5io-&gt;atread(&amp;expected_attr_int, attr_int_name);</div><div class="line">    h5io-&gt;atread(&amp;expected_attr_double, attr_double_name);</div><div class="line"></div><div class="line">    <span class="comment">// check results</span></div><div class="line">    <a class="code" href="../../db/de0/group__core__utils.html#gaf62bcd90f70e275191ab95136d85906b">CV_Assert</a>(<a class="code" href="../../dc/d84/group__core__basic.html#ga4e556cb8ad35a643a1ea66e035711bb9">norm</a>(attr_mat - expected_attr_mat) &lt; 1e-10);</div><div class="line">    <a class="code" href="../../db/de0/group__core__utils.html#gaf62bcd90f70e275191ab95136d85906b">CV_Assert</a>(attr_str.compare(expected_attr_str) == 0);</div><div class="line">    <a class="code" href="../../db/de0/group__core__utils.html#gaf62bcd90f70e275191ab95136d85906b">CV_Assert</a>(attr_int == expected_attr_int);</div><div class="line">    <a class="code" href="../../db/de0/group__core__utils.html#gaf62bcd90f70e275191ab95136d85906b">CV_Assert</a>(fabs(attr_double - expected_attr_double) &lt; 1e-10);</div><div class="line"></div><div class="line">    h5io-&gt;close();</div><div class="line">}</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main()</div><div class="line">{</div><div class="line">    read_write_attributes();</div><div class="line"></div><div class="line">    <span class="keywordflow">return</span> 0;</div><div class="line">}</div></div><!-- fragment --> <h2>Explanation </h2>
<p>The first step is to open the HDF5 file:</p>
<div class="fragment"><div class="line">    Ptr&lt;hdf::HDF5&gt; h5io = <a class="code" href="../../d6/dee/group__hdf5.html#ga243d7e303690af3c5c3686ca5785205e">hdf::open</a>(filename);</div></div><!-- fragment --><p> Then we use <a class="el" href="../../d3/d23/classcv_1_1hdf_1_1HDF5.html#adee759dc8dd95d6423d9a236527dacb8">cv::hdf::HDF5::atwrite()</a> to write attributes by specifying its value and name:</p>
<div class="fragment"><div class="line">    <a class="code" href="../../dc/d84/group__core__basic.html#ga1f6634802eeadfd7245bc75cf3e216c2">String</a> attr_mat_name = <span class="stringliteral">&quot;array attribute&quot;</span>;</div><div class="line">    Mat attr_mat;</div><div class="line">    attr_mat = (<a class="code" href="../../df/dfc/classcv_1_1Mat__.html">cv::Mat_&lt;float&gt;</a>(2, 3) &lt;&lt; 0, 1, 2, 3, 4, 5, 6);</div><div class="line">    <span class="keywordflow">if</span> (!h5io-&gt;atexists(attr_mat_name))</div><div class="line">        h5io-&gt;atwrite(attr_mat, attr_mat_name);</div></div><!-- fragment --> <dl class="section warning"><dt>Warning</dt><dd>Before writing an attribute, we have to make sure that the attribute does not exist using <a class="el" href="../../d3/d23/classcv_1_1hdf_1_1HDF5.html#a3ce1d93d746a79d6eabaadeae783956e">cv::hdf::HDF5::atexists()</a>.</dd></dl>
<p>To read an attribute, we use <a class="el" href="../../d3/d23/classcv_1_1hdf_1_1HDF5.html#a57d7777e74ec5dd83ee3c8533927dec8">cv::hdf::HDF5::atread()</a> by specifying the attribute name</p>
<div class="fragment"><div class="line">    h5io-&gt;atread(expected_attr_mat, attr_mat_name);</div></div><!-- fragment --><p> In the end, we have to close the HDF file</p>
<div class="fragment"><div class="line">    h5io-&gt;close();</div></div><!-- fragment --> <h2>Results </h2>
<p>Figure 1 and Figure 2 give the results visualized using the tool HDFView.</p>
<div class="image">
<img src="../../attributes-file.png" alt="attributes-file.png"/>
<div class="caption">
Figure 1: Attributes of the root group</div></div>
 <div class="image">
<img src="../../attributes-details.png" alt="attributes-details.png"/>
<div class="caption">
Figure 2: Detailed attribute information</div></div>
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